core: apply LogLevelMax to messages about units
    
    This commit applies the filtering imposed by LogLevelMax on a unit's
    processes to messages logged by PID1 about the unit as well.
    
    The target use case for this feature is a service that runs on a timer
    many times an hour, where the system administrator decides that writing
    a generic success message to the journal every few minutes or seconds
    adds no diagnostic value and isn't worth the clutter or disk I/O.
    
    (cherry picked from commit c2503e359a3a03b73acce0a92d27b1b8f54ff92c)